Cylinder deactivation may be used for fuel economy enhancement in a multi-cylinder, internal combustion engine wherein the intake valves are disabled in the closed position for selected cylinders, typically alternate cylinders in firing order. It is desirable to verify on such an engine that the cylinder deactivation system is working correctly: that is, that the cylinders are actually activated or de-activated in accordance with the commands of the de-activation control apparatus.
One method of verification known in the art is based on comparing a low-pass filtered (average) value of intake manifold absolute pressure (MAP) with an expected value based on engine operating conditions such as engine speed, load and cylinder de-activation mode, with a difference exceeding a threshold indicating an error. Although sound in theory, since the sensed value changes depending on how many cylinders are activated, it is difficult to make reliable determinations in practice due to two shortcomings. The first is in the determination of useful threshold values, since the ranges of expected values of filtered MAP for different numbers of activated cylinders in (1) properly functioning and (2) malfunctioning intake-side cylinder deactivation apparatus are often insufficiently spaced for reliable distinction and may even overlap. The second is that the signal time delay resulting from the low-pass filtering makes it difficult or impossible to determine the identity of the malfunctioning valve or valves.
The object of this invention is thus to provide a method for verifying cylinder activation status in an engine having selective intake valve activation that is not subject to the shortcomings mentioned above.